More than 70% of carious lesions (now over 125 million per year) are restored with acrylate-based resin composites which have a median service-life of less than 6 years (vs. 15 years for amalgam). The major reason for replacing these restorations is because of secondary caries, but the reason these lesion develop more rapidly on resin-based composites is unclear. Previous clinical studies on the relationship of the margin gap to the development of secondary caries have resulted in equivocal results due to the complexity of interacting variable and the inability to accurately monitor gap dimensions. More controlled in vitro experiments have focused on amalgams and not resin-based composites. This research will investigate a commercial acrylate-based resin dental composite and relevant polymer systems with the following specific aims: (1) to determine the role of the microgap in the development of secondary caries in human teeth, (2) to define how gap dimension controls the occluded chemistry within the gap, and (3) to determine how the occluded chemistry impacts the structure and properties of the resin and composite. This will be accomplished in a 4 year research effort with a multidisciplinary team and newly available measurement tools and techniques. Restorations with naturally formed microgaps and model restorations with precisely controlled microgap dimensions will be exposed to a realistic, controlled in vitro environment consisting of a consortium of oral bacteria relevant to caries in artificial saliva. The effects of the microbial biofilm on the tooth and the composite surface near the cavosurface margin and on the microgap dimensions will be monitored non-destructively with submicron resolution with a new interferometric technique, Super Resolution Vertical Scanning Interferometry. The relationship of gap dimensions, the activity of the different bacterial strains, and frequency and severity of secondary caries will be determined. In addition, the pH and Ca2+ concentration within the microgap will be measured as a function of depth and time with micro-sensor techniques that are new to dental research. Finally, the direct effect of the occluded microgap chemistry on the resin composite properties and chemistry will be examined using dynamic mechanical testing, fatigue measurements, and a newly developed minibeam technique, as well as FTIR to determine if there is direct evidence of backbone scission by hydrolysis. The successful completion of this research will conclusively test a leading theory for secondary caries in resin-based composites, critically examine the efficacy of these materials for use in dentistry, and develop new, more precise tools for biomaterial investigations in the laboratory and the clinic.